1. Field of the Invention
The present invention relates to an electrophotographic type image forming apparatus, such as facsimile machines and printers, which uses toner.
2. Description of the Related Art
In a conventional image forming apparatus, as it is illustrated in FIG. 13, such a configuration is known that a toner image T formed on a front surface of a photosensitive drum 1 is transferred onto a recording material S via an intermediate transfer belt 2. In a case of the illustrated example, since a full color image forming apparatus is used, a developing apparatus 3 includes four colors of development units 4Y, 4M, 4C, and 4k. Toner images of the respective colors are sequentially formed on the photosensitive drum 1, so that a full color image is formed on the intermediate transfer belt 2, and thereafter the full color image is transferred onto the recording material S. After the full color image is transferred onto the recording material S, the toner remaining on the intermediate transfer belt 2 is cleaned by causing a cleaning blade 5 to slide and to contact with the intermediate transfer belt 2.
In a case of the image forming apparatus including the intermediate transfer belt 2, a hollow defect that a central portion of an image is not transferred may occur. For example, as it is illustrated in FIG. 14, in a case where a vertical line image is formed in parallel with a recording material conveyance direction, a hollow portion tends to be larger as a line L becomes thinner. The hollow defect tends to occur since a material having a high degree of hardness, e.g., a fluorine resin, a polycarbonate resin, and a polyimide resin, is conventionally used as the intermediate transfer belt 2.
In other words, as it is illustrated in FIG. 15A, if the degree of hardness of the intermediate transfer belt 2 is high, when a layer of a toner t formed on the photosensitive drum 1 is transferred onto the intermediate transfer belt 2, a stress is concentrated onto the toner layer, so that plastic deformation of the toner layer is likely to occur. If the toner layer is plastically deformed, adhesion force (i.e., a real contract area) increases between toners t. If there is a source of roughness on the photosensitive drum 1 in the above state, the adhered toner t attaches at the source of roughness. As a result, the adhered toner t is not transferred onto the intermediate transfer belt 2, and the above described hollow defect occurs. The reason why the hollow portion becomes larger in a thinner line is that the thinner line tends to cause the toner layer to be suffered more stress concentration as described above.
To solve the above problem, as illustrated in FIG. 15B, an elastic layer having an appropriate elastic modulus is provided to the intermediate transfer belt 2 in order to relieve the stress concentrated to the toner t (specifically, of a vertical line image). Accordingly, reduction of the plastic deformation of the toner t can reduce the adhesion force between the toners t, and the hollow defect can be prevented. In a case where the intermediate transfer belt 2 having the above described elastic layer is cleaned by the cleaning blade 5 through a sliding-contacting with the intermediate transfer belt 2, as it is illustrated in FIG. 16A, the front surface of the intermediate transfer belt 2 is elastically deformed to reduce the toner t retained in the sliding-contact portion with the cleaning blade 5.
The toner t existing in the sliding-contact portion functions as a lubricant. Therefore, when the toner t retained in the sliding-contact portion comes to be less, the sliding ability between the intermediate transfer belt 2 and the cleaning blade 5 comes to be unstable. As a result thereof, a chatter mark or a fold occurs and a defect in cleaning tends to occur. On the other hand, in a case where a material having a high degree of hardness is used as the intermediate transfer belt 2, as it is illustrated in FIG. 16B, the toner t is retained more in the sliding-contact portion, so that the sliding ability between the intermediate transfer belt 2 and the cleaning blade 5 can be kept in a good condition except for a tendency of occurrence of the above described hollow defect.
Japanese Patent Application Laid-Open No. 2003-29550 discusses such a configuration that, as it is illustrated in FIG. 13, a lubricant application unit 6 is provided upstream the cleaning blade 5 in order to assist the sliding ability.
Recently, with respect to a request for space saving and cost reduction, a configuration including less number of parts in the image forming apparatus is desired. In a case of the configuration as discussed in the Japanese Patent Application Laid-Open No. 2003-29550, relying on the lubrication function by the lubricant application unit leads causes increase in a size of the lubricant application unit increases, i.e., the request for space saving cannot be satisfied.
In view of the above, more developed cleaning function of the cleaning blade 5 is required. In other words, the cleaning function of the cleaning blade 5 is required that the cleaning performance can be secured even in a case where the lubrication function of the lubricant application unit is low when the lubricant application unit is used.
To enhance the sliding ability between the cleaning blade 5 and the intermediate transfer belt 2, an approaching amount of the cleaning blade 5 with respect to the intermediate transfer belt 2 is required to be small. However, in order to lessen the approaching amount of the cleaning blade 5, an elastic modulus of the elastic layer of the intermediate transfer belt 2 should be raised to harden the elastic layer. In this case, the hollow defect, however, tends to occur for the reason as described above.